At what point in the electromagnetic spectrum, propagation stops !, well the answer is DC Zero Hertz, however at basement level, there still remains active energy, in the form of the Globe's Electric Charge, through the Sun's solar flare discharges, and the constant twenty four hour lightning that our Earth generates; this can be monitored, using a high gain amplifier and large surface aerial, would detect the ground to sky - potential charge of 120 Volts per metre.
Schumann Resonance waves, are generated by Lightning, and is similar to inducing a D.C. Voltage into a coil of x turns ratio; when monitored on an oscilloscope, you see the characteristic ringing effect, with large oscillations at first, reducing down to a dampened wave period, into infinity.
In the real world, D.C. impulse energy is supplied by lightning discharges on a global scale, because this is a constant 24 hour activity, standing waves are being propagated throughout the very low frequencies, from: 7.8 / 13.8 / 19.7 / 25.7 and 31.7 Htz rising beyond 32 Htz around and between, the Earth's circumference and its Ionosphere Cavity, as extremely long wave periods.
You might have noticed that these frequencies occur at wavelengths into sub audio.... To detect there E.M.Pulse/Wave, requires up-converting the periods, to hear the real - time signals... Commercial designs are not available, however there are individuals who take a dedicated interest in the sub-audio spectrum, producing there own circuit designs, although not particularly complex, they do require knowledge of electronics, and an ability to wind a three to five foot aerial coil, with (43,000 turns) on a 2"inch soft steel rod bundle, size is everything at these frequencies, then amplify the signals through a low noise head amp, followed by filtering of the spectrum, to notch out the A.C. mains grid system, and its associated rising harmonics.
For most of us, this is a bit of a problem, although the electronic processing is a small package, the aerial needs to be located outside, well away from the A.C. mains; open countries like America and Australia, where a quiet hum free location is less of a problem, here on this side of the pond we are faced with considerable E.M, pollution from the grid network, however there is an alternative option available to us.
Further up the spectrum, by taking a giant Hertzin leap, between the 1 to 10 Khz band the preferred spectrum for Whistler activity, being placed right in the audio spectrum makes detection that bit simpler, although we still need to use an aerial, long wire Electromagnetic E-Field, or loop Magnetic H-Field designs for signal detection, despite the fact its audio, the energy is electromagnetic, not in air sound pressure.
We enter a very strange world here, where unusual sounds occur; historically this band became noticed in the early 18th century period, when telephones and long wired - networks became widespread, the telephone operators who used to patch your call onto the other party, and monitor the calls, built up a list of odd and unidentified sounds into the earphones, today our knowledge of this phenomenon is better understood, still the scientific interest from, N.A.S.A. through the Inspire Project, shows that further investigation continues, using our Globe as the bench mark for other Worlds.
Lightning is the main source for VLF #, Very Low Frequency activity, but as in life there are other factors involved, and the interaction of our local star the sun, onto the Earths magnetic field, produce the sound of the "DAWN CHORUS", just like a rookery of birds, complex sounds with warbling tones, rising whistles of 0.1 to 0.2 second make this the most extraordinary sound detectable.
Many other sounds are: CLICKS, CHIRPS, CHINKS, TWEAKS, WARBLING, HISS, PINGS, and WHISTLES; a high pitched audio note from about 30 Khz decreasing to 350 Htz over a period of 0.3 to 5 seconds, with a pure note and smooth fall in frequency, all this comes about with a lightning discharge at a conjunction point, between the Global - hemispheres, produce a broad band of noise into the IONOSPHERE layers above the Earth, with its North South magnetic field poles providing long paths, where the reformed signals can travel; "TWEAKS" being very frequent propagation, can be a precursor to hearing a WHISTLER, however they require the right conditions to occur.
In addition modern products can QRM the band, after listening to the cacophony of electromagnetic emissions surrounding our very being, you realise just how polluted the spectrum is, most vehicles have alternator generators, there radiation and detection can be 30 feet away, in Jersey where I live, we have electric milk vehicles, every morning I can hear it arrive from 160 yards down the road, this is but one EM-Field along with, many interesting human and natural atmospheric sounds.
Since 1993 I have developed a number of Natural Radio receivers, having the prime requirement to listen via headphones at home, this is quite unheard of, in the realms of Natural Radio; the normal type of receiver being used is the Electric field receive: and has two problems in my estimation, it requires a long wire aerial or a modest whip aerial, and is impossible to use in 50/60 Htz mains proximity, due the hum and its rising harmonics, so outdoor operation is required; the other option is to use a H-Field loop aerial, where one can null out the unwanted hum, normally this type of aerial is large, around three feet in diameter, depending on the design used.
My solution was a simple design, using a long and medium wave radio FERRITE ROD aerial, tests and experiments produced good results and gain, over the 1 to 10 Khz band, by winding a small coil, centred onto the rod, wound directly over "sellotape" to protect the first turns; the coil layers are not critical, just wind back and forth between a 20mm area, using a sellotape strip to hold the start winding wire, and once wound.
The coil is tuned by two capacitors onto a step-up matching transformer, then pre-ampec by a BC109C low noise high gain transistor, the output is R.F, detected to the phones TDA7052 chip, being a tied-bridge amplifier output, giving 40db gain is adequate for Walkman type phones, of good quality with 103db sensitivity at 1Khz response.
An Emitter Follower transistor stage provides, microphone level Tape Recorder output, when using this facility, place the Recorder away from the receiver, to avoid motor - noise being detected; also keep in mind, induction principles are operating, and if the earphones come close to the aerial, then audio feedback will occur.
By following the layout, best results will be produced; when operating the receiver, you will notice a low rumbling noise while moving the electronics board around in - free space, due the transformer windings, detecting the Earths magnetic field, and being a normal by-product of this design.
A list of parts and the supplier code numbers are enclosed; please note I have not given any CASE or BOXING details, this is to keep the costs down, however by using an angled 90° section of wood or plastic, the PCB and Aerial can be mounted along with the volume control, battery and parts, onto the boards, angle section's being used.
GEO RECEPTION TIMES: Sunrise/Sunset is most active, you will need to experiment, as propagation of the Sun's Ionisation and your Global location, along with other lands overseas where storms or weather conditions even nearby, are constantly on the change; remember to take great care when listening; "Lightning by its very Nature is LOUD", keep Ear-Phone levels down, and save your hearing from damage.
A book called; Radio Science Observing: By Joseph J. Carr; ISBN 0-7906-1127-9: from Prompt Publications, covers the broad and concise area's of the VLF spectrum.
List of Parts for the Mk. 1a VLF receiver. Maplin order codes and prices are given but these may be obtained from other suppliers also.
STOCK DESCRIPTION ORDER CODE QUANTITY PRICE EACH TOTAL SRBP/PCB (100 X 74.1) JP47B 1 1-79 1-79 Battery Clip PP3 NE19V 1 -22 -22 Equipment Wire Red BL07H 1 10mtr Pack -69 -69 Equipment Wire Black BLOOA 1 10mtr Pack -69 -69 40 SWG, 50g Coil Wire BL60Q 1 1-64 1-64 Ferrite Rod 140 X 10mm YG22Y 1 1-29 1-29 Transformer LT-44 HX82D 1 1-19 1-19 Socket stereo 3.5mm FKZOW 2 -89 1-78 Headphones RZ93B 1 7-99 7-99 Pot 10K Lin + SW VQ13P 1 1-99 1-99 Control Knob/Red Line NC55K 1 -25 -25 IC TDA7052 UK79L 1 1-79 1-79 TRANSISTOR BC109C QB33L 2 -35 -70 LED 3.0mm 80mcd UK18U 1 -33 -33 DIODE IN4148 QL80B 1 -08 -08 CAPS 470µ AT33L 1 -24 -24 CAPS 100N VT64U 1 -22 -22 CAPS 47N WWZ0W 1 -19 -19 CAPS 22N BX72P 1 -15 -15 CAPS lON VT70M 2 -22 -44 CAPS 4N7 VT68Y 1 -22 -22 CAPS 2N2 RA40T 1 -16 -16 RESISTOR 3M3 M3M3 1 -16 -16 RESISTOR 1M M1M 2 -07 -14 RESISTOR 22K M22K 2 -07 -14 RESISTOR 10K M10K 1 -07 -07 RESISTOR 2K2 M2K2 1 -07 -07 RESISTOR 560R M560R 1 -07 -07
* For PCB wire links, use the cut-off's from Resistors and Caps *
Mount Aerial Rod Ends, onto Soft Door Foam, TAKE CARE the rod can BREAK if dropped
Mount Aerial Rod, in location to the PCB as shown, onto a wood or plastic base, with a right-angle section, to mount the Red LED, 10K Pot & Phone/Rec Sockets horizontally through the upright section. A plastic box would be more ideal !
A brief letter in reply, to your letter of the 3rd May, reference to my article on the Earths Lightning Phenomena.
I understand your need to review the points outlined, there condensed to provide an overview, to the very low spectrum of natural radio; its a fact, most people are unaware how electromagnetic propagation occurs, way down into Hertz/Cycles per-second, to almost D.C. even there an ultra slow variation of electric charge takes place around the Globe, through the positive. particle bursts from our local star the sun; with our daily rotation the interaction of these particles become ducted into the atmosphere, giving about 120 Volt background potential, per metre from the ground upwards, there no danger here, due to the very weak current involved; and pleased to say that, Everyday Practical Electronics, will have an article on this very subject this month.
My perspective and interests come from the detection and monitoring of these activities in reality we are enveloped and bombarded by low levels of Spheric Electromagnetic and charged pulses throughout our life, indeed considerable research has taken place, to define there effects on biological life, the results so far indicate we are influenced by there effects, particularly the brain and its electrical activity is subjected to temporal synchronisation, with the Earths 7 Htz natural cavity wave, that's resonating as a direct result from lightning, over a 24 hour period produces an averaged figure of 40,000 discharges.
I digress, but the subject is immense, and requires more than just a single letter, to explore its history; where many before myself, have monitored the very low bands in fact a publication from America called, The LOWDOWN; has a column devoted to it, alongside there main subject matter for beacons, from 1750 meter band and below to DC, most are privately built and operated, as well the military transmissions.